This invention relates to a control systems for servodrives of unknown dynamics having alternating inertial loads, Coriolis acceleration and unexpected disturbances. In particular, the present invention can be used in robotics, antenna and telescope drives, vehicle control and other systems. The main problems in designing control systems for servodrives of unknown dynamics are known to be (i) ensuring servosystem robustness, i.e. resistance or immunity to external disturbance; (ii) attaining high dynamic quality performance with respect to such factors as stability, degree of overshooting and transient process fading time; and (iii) minimizing power expenditure for the control and elimination of various vibratory effects and audible noise.
In most known methods and apparatuses for control of a servomotor, in particular where a direct current ("DC") motor is used (i.e. U.S. Pat. No. 4,749,927), the Pulse-Width Modulation ("PWM") method is commonly employed for generating servodrive control signals, where the duration of a control pulses .tau..sub.i is modulated in accordance with the current value of control error signal .DELTA.X.sub.i. Moreover, the pulse period T.sub.p is specified as constant during the control cycle T.sub.c and is usually selected in accordance with loss tolerance in the switching stages of the power amplifier and ability to satisfy the following relation: EQU T.sub.p &lt;K.sub.p T.sub.em
where:
K.sub.p --proportionality factor (8.ltoreq.K.sub.p .ltoreq.12), and
T.sub.em --electromotor electromechanical time constant satisfying EQU T.sub.em =(J.sub.p R.sub.r)/(C.sub.e /C.sub.m) (1)
where:
J.sub.p --equivalent moment of inertia about the electromotor rotation axis;
R.sub.r --electromotor rotor chain resistance;
C.sub.e, C.sub.m,--electric and magnetic constants.
The pulse amplitude "A" is also a selected constant and is equal to the nominal value of the electromotor power voltage. In some case, such an approach could provide sufficiently high dynamic quality performance when there is a constant load on the electromotor shaft. Even in this case, however, if A is constant, redundant power expenditure would be utilized for controlling the object.
It is known that in the functional diagram of digital control of a servodrive, the D.C. motor is usually considered as an aperiodic link that is equivalent to a Lowpass electromechanical Filter (LPF) with a transfer function of the following type: EQU W(P)=K.sub.m /(T.sub.em P+1) (2)
where:
K.sub.m --amplifying coefficient,
P--Laplace operator,
T.sub.em --electromechanical time constant.
When two consecutive pulses u(t) and v(t), with equal durations .tau. and spectrum densities S.sub.o (see FIG. 1) are applied to this Lowpass electromechanical Filter, the total energy E applied to the LPF input is shown by the following equation: ##EQU1## where:
E.sub.u, E.sub.v represent the energy of u(t) and v(t) signals respectively, and ##EQU2## is the interaction energy of the signals
It is known, that the high frequency portion of the signals' interaction energy usually reduces the quality of the transient processes preceding in the electric drive in that the degree of overshooting and the transient process fading time increase and parasitic vibrations and noise disturbances appear, especially at low velocities. FIG. 2 depicts the impulse response of the LPF over time, when signals u(t) and v(t) are applied.
When a traditional PWM is used, the above effects increase even more as duration of the controlling pulses is decreasing when the servodrive approaches the preset control point, and when in accordance with the Indeterminacy Principle, w.sub.u .tau..gtoreq.1/.pi., the pulse frequency spectrum is extended. Hence, the smaller part of applied energy concentrates in the transmission band of a LPF. For an electric drive this means that the part of the useful energy which provides the motor rotation decreases and the parasitic effects (e.g., thermal, vibratory, noise) increase. This process is especially aggravated when the moment of inertia about the motor shaft changes due to the load changes, which causes corresponding changes of the transmission band of the LPF. In this case the robustness of the system is also not provided.
Moreover, when the pulse duration is changing at constant values of T.sub.p and A, the current ripple coefficient of the electromotor also changes, which causes velocity pulsation.
There have been attempts to solve the above problems through the addition of various means, e.g. the addition of a feedforward control as in U.S. Pat. No. 5,107,193. These attempts, however, do not maintain the robustness of the system and do not decrease power expenditure utilized for control of the unknown movement dynamics of the controlled object, since a traditional PWM with constant T.sub.p and A is also utilized in that case (U.S. Pat. No. 5,107,193).